Arrival Of Seismic Waves Research Articles

Positive electrification on the floor of an underground mine gallery at the arrival of seismic waves and similar electrification on the surface of partially stressed rocks in laboratory

Terra Nova, 22, 203–207, 2010 Abstract A large plate electrode in a mine gallery detected electric signals when seismic waves arrived. This signal is equivalent to the generation of an electric field with the upward negative and the downward positive. One possible reason for detection of the electric field is a positive electrification of the floor. Here, we studied such electrification in the laboratory using an andesite block quarried from the same mine. When partially loaded at one end, the block exhibits positive electrification on the surface of the other unstressed end. The activation of positive hole charge carriers reasonably explain the laboratory results. The positive holes are activated by breaking peroxy bonds, a common type of lattice defects in igneous rock-forming minerals. The positive holes spread to the surface, thereby causing the electric field. The same charge carriers may be activated upon arrival of seismic waves.

New algorithms of emission tomography for passive seismic monitoring of a producing hydrocarbon deposit: Part II. Results of real data processing

The new suggested algorithms of seismic emission tomography and those used earlier made it possible to dispose of the effects caused by a strong industrial noise, to study the structure of the upper part of the Earth’s crust and to monitor the seismic processes that take place within the oil deposit being developed in West Siberia. An efficient use of the industrial background seismic illumination allowed three-dimensional imaging of the fault zone. From the recorded seismic noise at the time of arrival of the high-amplitude low-frequency seismic wave, a localized soliton-like object floating upwards on the rupture with the velocity of 10 m/s is identified. In the depth range from 1 to 6 km a vertical filament-like seismic source that is hypothesized to be associated with the vortex geological structure, namely, the zone of torsional strains, is revealed. The position of this source in a plane remains stable within 9 days, although the bursts in the amplitude of both industrial and natural seismic radiation affect the depth distribution of the radiation and result in the appearance of additional seismic sources.

The anomalies of Fedchenko astronomic clocks readings before some large earthquakes

On an instrumental complex of SRI NAO, by anomalous great deviations on the course of Fedchenko clock, seismic waves are revealed caused by some large earthquakes (the Kurils, Pakistan, Indonesia, the Molucca Sea, Turkey, Greece, etc). Several minutes before a seismic wave arrival, decrease of dispersions of Fedchenko astronomical clock readings presumably caused of an infra low frequency electromagnetic radiation from an epicenter of earthquake was observed.

Earthquake triggering of mud volcanoes

Mud volcanoes sometimes erupt within days after nearby earthquakes. The number of such nearly coincident events is larger than would be expected by chance and the eruptions are thus assumed to be triggered by earthquakes. Here we compile observations of the response of mud volcanoes and other geologic systems (earthquakes, volcanoes, liquefaction, ground water, and geysers) to earthquakes. The compilation shows a clear magnitude–distance threshold for triggering, suggesting that these seemingly disparate phenomena may share similar underlying triggering mechanisms. The compilation also shows that pre-existing geysers and already-erupting volcanoes and mud volcanoes are much more sensitive to earthquakes than quiescent systems. Several changes produced by earthquakes have been proposed as triggering mechanisms, including liquefaction and loss of strength, increased hydraulic permeability or removing hydraulic barriers, and bubble nucleation and growth. We present new measurements of the response of erupted mud samples to oscillatory shear at seismic frequencies and amplitudes, and find that loss of strength occurs at strain amplitudes greater than 10 −3. This is much larger than the peak dynamic strains associated with earthquakes that may have triggered eruptions or influenced already-erupting mud volcanoes. Therefore, we do not favor loss of strength as a general triggering mechanism. Mechanisms involving bubbles require significant supersaturation or incompressible mud, and neither condition is likely to be relevant. We analyze the response of the Niikappu group of mud volcanoes in Japan to several earthquakes. We find that this system is insensitive to earthquakes if an eruption has occurred within the previous couple of years, and that static strain magnitudes are very small and not correlated with triggering suggesting that triggering likely results from dynamic strain. Moreover, triggering may be frequency-dependent with longer period seismic waves being more effective at triggering. Available data are insufficient, however, to determine whether triggering characteristics at Niikappu are representative of triggered eruptions in general. Nor can we determine the exact mechanism by which dynamic (long-period) strains induce eruption, but given the apparent failure of all mechanisms except increasing permeability and breaching barriers we favor these. More observations and longer records are needed. In particular, gas measurements and broadband seismic data can be collected remotely and continuously, and provide key information about processes that occur during and immediately after the arrival of seismic waves.

Using SeisMac to Turn Your Laptop into a Seismograph for Teaching

You probably are familiar with the situation. You're trying to explain the arrival of seismic waves to your class. Or maybe it is your spouse (or child, grandparent, parole officer, whatever). You are excited and enthusiastic, talking about frequencies and wavelengths, dispersion, Hilbert transforms, and so on, and all you get in return is blank stares. They don't get it. The problem is that you are showing them a static black-and-white image of squiggly lines, and you just can't get them to see the dynamic motions and transfers of energy involved. You wish you had an actual seismometer to whip out and use to demonstrate your points. Well, your wish has been granted! In fact, you very well may have access to it already. SeisMac is a free Mac OS X application that runs on a MacBook or MacBook Pro and turns built-in accelerometers into a three-component seismograph. These Mac laptops are installed with a sudden-motion sensor, similar to automobile air-bag accelerometers, that is designed to lock up your hard drive if your laptop falls to the ground. It works. Trust us (or, rather, trust the dents in our laptops). Early on, Mac users discovered this feature and were using it to do things like turn their laptops into Star Wars light sabers (complete with sound effects). However, freelance programmer Daniel Griscom realized the potential and created SeisMac (which he wrote about in Electronic Seismologist for SRL volume 77, issue 6 (November/December 2006) pp. 731–733). With some seed funding and guidance from the Incorporated Research Institutions for Seismology (IRIS), SeisMac was honed into a real seismograph. Yes, of course, you could achieve the same results by purchasing a separate three-component accelerometer and developing a graphical user interface (GUI) that could display the signal in a user-friendly manner. But most likely, …

Signal‐based ray tracing modeling in complex tectonics

The objective of this talk is to present a novel method which can perform the fast computation of the times of arrival of seismic waves which propagate between a source and an array of receivers in a stratified medium. This method combines signal processing concepts for the approximation of interfaces and wavefronts, and ray theory for the propagation of wavefronts. The main idea is to put in adequation the computations with the the precision and the resolution of the source signal in order to avoid unnecessary computational effort. This new approach leads to the redefinition and simplification of the model through which waves propagate. The modifications are governed by the spectral characteristics of the source signal. All rays are computed without any omission at a much lower cost in computing time than classical methods. In addition, we will show how to include surface waves such as head waves within the proposed method.

Investigation of underground cavities in a two‐layer model using the refraction seismic method

ABSTRACTThis paper presents a new approach to the investigation of underground cavities. Our technique is based on the refraction seismic method. We have studied a two‐dimensional, two‐layer geological model. In our model, the lower seismic velocity layer is situated above the higher seismic velocity layer, with a circular cavity positioned within the upper layer. We have investigated the influence exerted by the cavity on the first arrivals of seismic waves. The obtained traveltimes are solutions of the eikonal equation and are presented using the time‐distance graph. All refracted waves encountering the cavity have to circumvent it, as it represents an impediment to the propagation of seismic energy. This circumvention causes delays in the first arrivals of the seismic waves at the surface as compared to traveltimes with no cavity present. These delays create a characteristic shape of the time‐distance graph, characterized by the peak point in which the plot line has a discontinuous change. Using this graph and analysing the delays of the first arrivals, we have derived expressions for determining both the position and size of the circular cavity. The practical application of the derived relations has been tested on a model test site built in a natural rock setting. This simple method indicates the presence of the cavity. The accuracy of the calculated cavity parameters: the horizontal position , the depth and cavity radius depends on the geophone spacing. For geophone spacing equal to or less than the cavity radius, the accuracy of the method is shown to be acceptable. The maximal estimated error is equal to a half geophone spacing. Finally, we have demonstrated that this method is also applicable in the detection of non‐circular cavities.

Atmospheric pressure change associated with the 2003 Tokachi‐Oki earthquake

Clear atmospheric pressure changes associated with the 2003 Tokachi‐Oki, Japan, earthquake with Mw 8.3 were recorded with the microbarographs distributed in Japan. The pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves, suggesting that the observed pressure change was driven by the ground motion of seismic waves passing by the site. We computed the seismic‐to‐pressure transfer function (i.e., the spectral ratio of the pressure change to the vertical ground motion velocity) for periods between 10 to 50 s from the co‐located barograph and seismograph records. Comparison of the observed transfer function with the theoretical one including the finite frequency and wavelength effects for a gravitationally stratified isothermal atmosphere confirms that the observed amplitude and phase of the pressure change are explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors.

Rapid determination of earthquake magnitude using GPS for tsunami warning systems

The 26 December 2004 Sumatra earthquake (Mw 9.2–9.3) generated the most deadly tsunami in history. Yet within the first hour, the true danger of a major oceanwide tsunami was not indicated by seismic magnitude estimates, which were far too low (Mw 8.0–8.5). This problem relates to the inherent saturation of early seismic‐wave methods. Here we show that the earthquake's true size and tsunami potential can be determined using Global Positioning System (GPS) data up to only 15 min after earthquake initiation, by tracking the mean displacement of the Earth's surface associated with the arrival of seismic waves. Within minutes, displacements of >10 mm are detectable as far away as India, consistent with results using weeks of data after the event. These displacements imply Mw 9.0 ± 0.1, indicating a high tsunami potential. This suggests existing GPS infrastructure could be developed into an effective component of tsunami warning systems.

Seismomagnetic signals associated with Sakhalin earthquakes

Results of magnetic observations in Yuzhno-Sakhalinsk during a strong earthquake and a local swarm of earthquakes, as well as magnetic signals recorded during propagation of seismic waves, are considered. The possibility of the identification of these signals in magnetograms is discussed. The results of this study are shown to be significant for the development of a prompt method for warning of the arrival of seismic waves.

Dynamic response of frequent tremors at Aso volcano to teleseismic waves from the 1999 Chi-Chi, Taiwan earthquake

Teleseismic waves from the 1999 Chi-Chi, Taiwan earthquake dynamically triggered isolated tremor and micro-earthquakes at Aso volcano, Japan, which is at a distance of 1400 km. Immediately after the arrival of the P-waves, short-period tremor (SPT) began to occur more frequently for about 2000 s, and during and after the arrival of surface waves, 2 micro-earthquakes were observed around the crater. Observations of frequent SPT are an indication of triggered isolated tremor which consists of both SPT and long-period tremor (LPT) components. For the cases of arrivals of seismic waves from other large teleseisms from March 1995 to June 2002, triggering of isolated tremor was observed during 1998–1999. During this period increased thermal supply to the crater was observed while the background activity was relatively low, and the volcanic system beneath the crater was more responsive to external stress perturbations.

Seismic dynamo effects associated with the M7.1 earthquake of 26 May 2003 off Miyagi Prefecture and the M6.4 earthquake of 26 July 2003 in northern Miyagi Prefecture, NE Japan

An earthquake of M7.1 occurred off Miyagi Prefecture, NE Japan, on May 26, 2003 at the depth of about 70 km. Just two months later, on July 26, 2003, another shallow earthquake of M6.4 occurred in northern Miyagi Prefecture. In order to investigate whether small signals precursory to the arrival of seismic wave appeared for these earthquakes, we examined the MT records which have continuously been acquired at two stations, Mizusawa and Esashi of the Geographical Survey Institute, located about 50 km northwest of the epicenter of the M7.1 earthquake and about 100 km north of the M6.4 earthquake. Unfortunately, seismometers were not available at these stations, and hence direct comparison between the MT and the seismic records were not possible. We therefore refer to the seismic record obtained at a nearby station belonging to the nation-wide seismic network called ‘Hi-net’ and also we used the arrival times estimated by Japan Meteorological Agency (JMA) for these MT stations. In conclusion, no clear signals preceding the arrival of seismic wave could be detected, although the possibility is suggested that a slight signal may have appeared in one magnetic component at the Mizusawa station before the arrival of seismic wave for the M7.1 earthquake.

The Ohio Seismic Network

In January 1999, the Ohio Seismic Network (OhioSeis) began operations with an initial deployment of 15 stations at colleges, universities, and other institutions in various parts of the state. Funding for the network was provided by the Ohio Emergency Management Agency with Federal Emergency Management Agency Funds for earthquake mitigation. The Ohio Geological Survey assumed the responsibility for organizing, managing, and coordinating the network, and data analysis. The primary mission of the network is to detect and precisely locate earthquakes within the state’s borders, report the severity and potential damage from an event to emergency response officials, and to rapidly respond to media and citizen inquiries after a felt earthquake. Both the media and the public have quickly become dependant on the Ohio Seismic Network to be the primary source of information after an event. Secondary but important goals of both the Ohio Geological Survey and the Ohio Emergency Management Agency are to evaluate seismic risk and to educate citizens about earthquake hazard in the state. The establishment of seismic stations at various educational institutions has provided a valuable experience to students who commonly witness real-time arrival of seismic waves from large teleseismic events. Many of the geology instructors who host stations have developed educational exercises on earthquakes based on the seismic events recorded at their stations and several in-state educational institutions have developed student exercises based on the information available on the Ohio Seismic Network Web site. There has been the added educational bonus of local-station media attention after a local felt earthquake or a devastating teleseismic event. This has brought positive publicity to the local host institution and generated a sense of pride and earthquake awareness in the local community. The public visibility of the Ohio Seismic Network has increased earthquake awareness in the state and has been an integral factor in educating citizens about seismic risk. The extensive OhioSeis Web site (http://www.ohiodnr.com/ OhioSeis) includes generalized educational information, technical information on recent local and regional earthquakes, an extensive chronological catalog of in-state and border-region events, epicenter and basement structure maps of Ohio, and a link to the U.S. Geological Survey

Small electric and magnetic signals observed before the arrival of seismic wave

Electric and magnetic data were obtained above the focal area in association with the 1999 Izmit, Turkey earthquake. The acquired data are extremely important for studies of electromagnetic phenomena associated with earthquakes, which have attracted much attention even without clear physical understanding of their characteristics. We have already reported that large electric and magnetic variations observed during the earthquake were simply due to seismic waves through the mechanism of seismic dynamo effect, because they appeared neither before nor simultaneously with the origin time of the earthquake but a few seconds later, with the arrival of seismic wave. In this letter we show the result of our further analyses. Our detailed examination of the electric and magnetic data disclosed small signals appearing less than one second before the large signals associated with the seismic waves. It is not yet solved whether this observational fact is simply one aspect of the seismic dynamo effect or requires a new mechanism.

Fault Slip, Seismicity, and Deformation in Mexicali Valley, Baja California, Mexico, after the M 7.1 1999 Hector Mine Earthquake

A local small deformation network in Mexicali Valley (Baja California, Mexico) recorded fault slip, seismicity, and deformation produced by the 16 October 1999 M 7.1 Hector Mine earthquake 260 km away. The network consists of two crackmeters, two biaxial surface tiltmeters, and one broadband seismograph, all recording continuously; these instruments are complemented by other seismological instruments within the valley. Local microearthquakes with clear P and S phases began 2 hours after arrival of the Hector Mine P wave, and earthquakes with M >2.5 (large enough to be located) started about 33 hours after the earthquake. Local seismicity continued, with decreasing frequency, for about 2 weeks after the Hector Mine earthquake. Vertical and horizontal slip and tilt began on the Imperial fault simultaneously with the Hector Mine earthquake seismic wave arrival, which is clearly recorded by the tiltmeters. Deformation recorded by a tiltmeter on the Cerro Prieto geothermal field began about 30 hours after the main event and ended 13 hours later after the occurrence of the largest local earthquake ( M 4.1) in this episode. The temporal pattern of seismicity and deformation suggests that deformation is the main process triggered by the Hector Mine earthquake waves in the study area and the transient local seismicity increase is a response to the triggered deformation.

The Athens 1999 mainshock (Mw=5.9)and the evolution of its aftershock sequence

The spatial distribution of the aftershocks that followed the September 1999 mainshock (Mw=5.9), which caused severe damage and loss of life in the nearby city of Athens, is examined in the present work. Ρ and S arrivals of seismic waves recorded by the permanent seismic network as well as by a number of digital seismographs and accelerographs, which had been deployed in the broader epicentral area shortly after the mainshock occurrence, were used for the determination of the focal parameters of the mainshock and its aftershocks. The spatial distribution of the aftershocks led to the recognition of the fault, which produced the September mainshock, while certain features of the rupture process may be deduced on the basis of their spatiotemporal variation

Co‐seismic geoelectric potential changes observed in Japan

Co‐seismic signals of geoelectric potential have been observed for several earthquakes (EQs) in Japan. It has been argued that if there are precursory changes, stronger signals should be at the time of EQ. Our results prove that co‐seismic signals do exist, but they start with the arrival of seismic waves and not at the origin time of EQs. Their amplitude does not scale with the dipole length. Both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source. Observed changes contain two components; oscillatory and offset/decay ones. The former is synchronized with seismic waves, whereas the latter, with a rise time of a few seconds and a decay time of 0.1–1 hour, is suspected to be related to EQ induced subsurface hydrological change. In both cases, the origin of electric potential variations may be electrokinetic.

Topography of the transition zone seismic discontinuities

Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. Thermodynamic principles govern phase transformations, making them sensitive to changes in the mantle's ambient conditions through the thermodynamic response: Changes in temperature or composition shift the transformation to a different pressure, creating topography on a level discontinuity. With this use as an exploratory tool for the mantle in mind, we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities. Reviewing the evidence for and against the phase transition model, we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone.

Field experimentation on the detectability of co-seismic electric signals

Simultaneous electric, magnetic and seismic measurements in Greece show that, for epicentral distances of the order of 100km and for M6 class earthquakes, no electric disturbances in the frequency range DC to 50Hz are recorded at earthquake origin time. Electric disturbances are recorded only during the arrival of seismic waves and have the following properties: (a) The amplitude does not scale with the dipole length and may significantly differ for parallel horizontal short dipoles located at neighboring sites. (b) The amplitude of the vertical component does not exceed the strongest horizontal one. (c) The data collected with a sampling rate of 50Hz, show that strong frequency content lies in the range up to around 2.5 Hz and the disturbances vanish at frequencies less than 0.1Hz and more than 5Hz, providing an explanation on the absence of these disturbances in the usual VAN real time records. These disturbances are accompanied with magnetic variations recorded either by coil and/or by torsion photoelectric magnetometers. It is not yet clear whether the magnetic variations are solely due, or not, to“vibrations”of the magnetometers upon the arrival of seismic waves. Electric disturbances were also recorded upon the arrival of seismic waves of the Izmit, Turkey, earthquake of August 17, 1999 in most of our stations. However, no pronounced electric disturbances were recorded during the Athens earthquake of September 7, 1999, neither for the Turkish event of November 12, 1999. The basic results of this paper are compatible with those reported by Uyeda et al.1)

Seismic Velocity and Density Jumps Across the 410- and 660-Kilometer Discontinuities.

The average seismic velocity and density jumps across the 410- and 660-kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long-period seismic wave arrivals that reflect off of these interfaces. The preliminary reference Earth model (PREM) is within the computed 95 percent confidence ellipse for the 410-km discontinuity but outside the allowed jumps across the 660-kilometer discontinuity. Current pyrolite mantle models appear consistent with the constraints for the 410-kilometer discontinuity but overpredict amplitudes for the 660-kilometer reflections. The density jump across the 660-kilometer discontinuity is between 4 and 6 percent, below the PREM value of 9.3 percent commonly used in mantle convection calculations.